The present invention relates to water treatment chemical formulations, and more particularly to chemical formulations used to treat water containing halogens as sanitizing agents. The water treatment chemical formulations contain cyanuric acid and an alkali metal molybdate or silicate as an anticorrosion agent.
The corrosion of metal equipment is a widespread and expensive problem in many industrial settings such as open recirculating cooling towers and in the agricultural industry in fertilizer storage and irrigation systems, as well as in consumer settings such as swimming pools and spas. Expenses arise due to corrosion from costly repairs and replacement of metal equipment parts that have been weakened or destroyed by the action of water and oxygen at the metal surface.
Pitting corrosion is a localized form of corrosion by which cavities or holes are produced in a metal. Pitting is commonly observed on surfaces with little or no general corrosion. Pitting typically occurs as a process of local anodic dissolution where metal loss is exacerbated by the presence of a small anode and a large cathode. Pitting is often found in situations where resistance against general corrosion is conferred by passive surface films. Localized pitting attack is found where these passive films have broken down. Pitting corrosion is generally of greater concern than uniform corrosion because it is more difficult to detect and protect against. Corrosion products often cover the pits, making them difficult to identify. Apart from localized loss of thickness at the metal surface, corrosion pits can also be harmful by acting as stress risers. Corrosion pits are commonly the starting points for cracking and fatigue.
An extremely corrosive microenvironment typically forms within a corrosion pit that varies considerably from the bulk corrosive environment. For example, when stainless steels undergo electrochemical pitting processes in water containing chloride ions, a microenvironment having a high concentration of hydrochloric acid (and thus low pH) typically forms within the pits. This corrosive microenvironment can hasten growth of pits once initially formed.
Pitting corrosion can produce pits in a variety of configurations. For example, open pits may be formed, or pits may be covered with a semi-permeable skin comprising corrosion products. Pits can be hemispherical or cup-shaped, flat-walled, or completely irregular in shape. Pits may also reveal the crystal structure of the metal. Trough-shaped pits may be narrow and deep or shallow and wide. Sideways pits may be subsurface, undercutting, or attack the grain of the metal horizontally.
Pitting is quantified in various ways. The pitting factor is the ratio of the depth of the deepest corrosion pit divided by the average penetration as calculated from weight loss. The Pitting Resistance Equivalent Number (PREN) is an empirical relationship used to predict the pitting resistance of austenitic and duplex stainless steels, and is defined by the equation PREN=Cr+3.3(Mo+0.5 W)+16N.
The extent of pitting corrosion can vary greatly depending on the exposure conditions and surface condition of the material. Commonly used methods to determine the pitting corrosion resistance include ASTM G (standard reference test method for making poteniostatic and potentiodynamic anodic polarization measurements), ASTM G-46 (practice for examination and evaluation of pitting corrosion), ASTM G-48 (test methods for pitting and crevice corrosion resistance of stainless steels and related alloys by the use of ferric chloride solution), ASTM G-61 (cyclic potentiodynamic polarization measurements for localized corrosion susceptibility of iron, nickel or cobalt based alloys), ASTM G-85 (modified salt spray testing), and NACE TM0274 (dynamic corrosion testing of metals in high temperature water).
Visual examination and metallographic techniques are particularly useful in characterizing the physical nature of the localized corrosive attack. The most relevant information is generally the maximum attack depth or rate since these parameters will most directly indicate the serviceability of the metal component.
Methods of preventing or retarding pitting include, amongst others, increasing the velocity at which the liquid in contact with the exposed metal moves across the metal surface; removing scale and other solids deposits from the metal surface; employing ferritic metal alloys with higher alloy content (for example, chromium, molybdenum, and nitrogen), expressed as a higher PREN number; use of titanium or zirconium alloys; dearating the environment at the exposed metal surface; reducing the temperature of the system; employing inert or purge gasses in the system; maintaining the system in an alkaline state (pH greater than 7); and employing corrosion inhibitors to enhance resistance of the metal to corrosion pitting.
Of the above-mentioned methods for preventing or retarding pitting corrosion, the use of corrosion inhibitors as additives to aqueous systems in contact with exposed metal surfaces is widely used. Corrosion inhibitors typically inhibit the rate of corrosion at a metal surface by forming a passivation layer on the metal surface, namely, a surface film that physically blocks the diffusion of ions to and from the surface of the metal. Such passivation films may be produced by deposition of the corrosion inhibitor on the metal surface, or by reaction of the corrosion inhibitor with the metal surface. Other corrosion inhibitors act by neutralizing corrosion-causing components present in the aqueous system.
A corrosion inhibitor system capable of preventing or retarding pitting corrosion while providing satisfactory performance in other aspects of water treatment is highly desirable. Corrosion inhibitor systems, wherein the systems are provided that incorporate a combination of cyanuric acid and either a silicate, a molybdate, or both a silicate and molybdate. This combination of components provides superior protection against pitting corrosion in ferritic metals.